Gas mixture and use thereof for people to breathe as required in the event of pressure drops in aircraft or in the event of hyperventilation, and method therefor

ABSTRACT

Gas mixture used for ventilation of passengers and crew in emergency situations. Depending on the density altitude, it has 7±5% CO 2  at 15,000 ft flying altitude increasing to 17±5% CO 2  at 30,000 ft flying altitude. The carbon dioxide improves the bioavailability of oxygen in the body. The gas mixture is produced by additive dosage of CO 2  to either pure O 2  or to a gas mixture having a fraction of N 2  and a fraction of O 2 . The method for ensuring good ventilation in case of loss of cabin pressure, or generally in case of hyperventilation, involves making the gas mixture above available via respiration masks. The use of such a gas mixture also for ensuring good ventilation of people with limited mobility, if such ventilation is required. The prescribed amount of onboard oxygen for aircraft can thus be reduced and flight routes leading directly over high-altitude terrain may be taken.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/EP2015/080770 filed Dec. 21, 2015, claiming priority based on SwissPatent Application No. 02028/14 filed Dec. 24, 2014, the contents of allof which are incorporated herein by reference in their entirety.

The invention relates to a gas mixture as product and to the use of saidgas mixture for artificial respiration of human beings as needed at highdensity altitudes. The invention further relates to a method for makingthe gas mixture available for artificial respiration. In particular, itrelates to the use of a certain gas mixture as a product for artificialrespiration in case of cabin depressurization in aircraft to ensuresufficient oxygen saturation of air passengers and crew, and to providesupport in case of insufficient or lack of spontaneous breathing andhyperventilation, respectively.

A typical oxygen saturation is immanent in the human body, the valuebeing dependent on age, clinical picture, and also on particularcircumstances. The value indicates the fraction of oxygen-saturatedhaemoglobin in the blood, which provides information on the efficiencyof breathing and oxygen transport in the body. Oxygen under-saturationin human beings arises either because the oxygen partial pressure in theenvironment is too low (corresponding to a height above 10,000 ft or3048 m, for example) and/or as a consequence of health impairment. Asthere are different causes, the medical treatments vary equally. Ingeneral, a distinction is made between assisted ventilation andcontrolled (mandatory) ventilation. In the case of assisted ventilation,a ventilator machine has a purely support function to assistinsufficient spontaneous breathing. The patient breaths himself andcontrols the breathing rate. By contrast, in case of controlledventilation, the ventilator machine completely replaces the endogenousbreathing function. The oxygen concentration of the artificiallysupplied air can be adjusted between a normal level concentration of 21%in the gas mixture, up to 100%, according to requirements. The inspiredoxygen fraction is identified by FiO₂ (fraction of inspired oxygen). Itis known that administering a FiO₂ higher than 0.5 (equivalent to anoxygen fraction of 50% in the respiratory air) for a prolonged periodhas harmful effects. Oxygen is a powerful oxidizing agent which alsooxidizes other substances in the blood besides haemoglobin. However,enzymes in the body reverse this oxidation process. On the other hand,if pure oxygen is supplied to the body beyond a certain period of time,the “methaemoglobin reductase” cannot repair this damage given of theincreased oxidation of haemoglobin and other proteins. When theendogenous anti-oxidation system is exhausted, the oxygen radicalsreleased cause oxygen toxicity which is manifested in effects on thecentral nervous system, the lungs and vision. Nonetheless, patients in alife-threatening condition are ventilated—if only temporarily—with pureoxygen, i.e. with FiO₂=1. During pre-oxygenation as well, i.e. theprophylactic enrichment of the oxygen reservoir of the lungs—accordingto usual practice before anaesthetic induction for example—is performedby supplying 100% oxygen to the patient to flush the nitrogen of tidalair from the respiratory tract. In the same way, crew and passengers areventilated with pure oxygen in the case of loss of cabin pressure inaircraft. Here, the prevailing argument is that a large quantity ofoxygen should be introduced into body tissue as quickly as possible, asthe occupants of the aircraft might already be suffering from anundersupply of oxygen. In this context, any adverse effects aretherefore obviated or accepted.

However, the risks of ventilation are not attributable solely to theproperties of oxygen. The concentration of carbon dioxide (CO2) in the(arterial) bloodstream also plays a critical role. Breathing control andregulation is primarily effected via chemoreceptors or chemosensors thatare sensitive to the partial pressure of carbon dioxide (in thisrespect, oxygen-sensitive receptors and other receptors are lessimportant). The level of carbon dioxide in the blood is thus avegetative stimulus for breathing regulation. If the level of carbondioxide in the blood exceeds a characteristic threshold value,respiratory stimulants are released. Conversely, in case ofhyperventilation and the concomitant reduction of carbon dioxide partialpressure in the blood (hypocapnia), a reflex restriction is imposed onrespiration. Thus, in order to suppress their breathing reflex,inexperienced divers often hyperventilate in order to exhale carbondioxide and consequently remain under water for a longer time. However,this involves considerable risks, possibly resulting in a loss ofconsciousness and in the case of diving—drowning (known as shallow waterblackout). As outlined in detail further below, symptoms of deficiencyoccur with decreasing endogenous carbon dioxide level, from unpleasantto life-threatening. Limited freedom of movement further amplifies theeffect of hyperventilation, leading to a further decrease of the carbondioxide level in blood. The reason for this is that the muscles thenproduce less endogenous carbon dioxide. Consequently, the symptoms ofcarbon dioxide deficiency occur more quickly and more severely. Thisproblem is typical for the situation of loss of cabin pressure inaircraft, because passengers necessarily only have limited mobility.Thus, monitoring the carbon dioxide level in such a situation isimperative during ventilation, e.g. by measuring the end-expiratorypartial pressure of carbon dioxide.

If the cabin pressure in an aircraft drops below a critical value, theceiling compartments above the passengers' seats open and oxygen masksdrop from their holders and are suspended in front of the passengers'faces. Pure oxygen flows into the masks through a supply tube and isthen inhaled into the nose and mouth of the breathing person. The oxygensupplied to the passengers is either generated in chemical oxygengenerators or carried in pressurized on-board gas cylinders, whereas theoxygen supplied to the pilots in the cockpit is released from a separatepressurized gas system. The amount of oxygen stored in an aircraftdepends on the certification and intended use of the aircraft and alsoon the routes it is intended to fly. In this context, consideration mustbe given to whether the aircraft will fly mainly over land or sea, orwhether it is designed to fly long distances over high-altitude terrainand mountainous areas.

The amount of time within which an individual remains able to performflying duties efficiently while exposed to an environment ofinsufficient oxygen supply is referred to as time of usefulconsciousness (TUC) or Effective Performance Time (EPT). After thistime, the body tissue and organs suffer from significant undersupply ofoxygen and the body becomes hypoxic. Below a certain oxygen saturationof the brain, the ability to act rationally is lost, followed by loss ofconsciousness. The TUC is specified for a given flight level (the termflight level refers to a level of equal barometric pressure,corresponding to a given flying altitude expressed in hundreds of feet)and decreases with increasing flying altitude. The table below shows theaverage TUCs for various flight levels.

Flight Level TUC Altitude in Meters Altitude in Feet FL 150 ≥30 min4,572 15,000 FL 180 20-30 min 5,486 18,000 FL 220 5-10 min 6,705 22,000FL 250 3-6 min 7,620 25,000 FL 280 2.5-3 min 8,534 28,000 FL 300 1-3 min9,144 30,000 FL 350 30-60 sec 10,668 35,000 FL 400 15-20 sec 12,19240,000 FL 430 9-15 sec 13,106 43,000 ≥FL 500 6-9 sec 15,240 50,000

The speed of decompression also affects the TUC: the more rapid thedecompression, the shorter the TUC. This is why, given the rapidlyoccurring undersupply of oxygen to the body, rapid replacement of theoxygen supply is essential for survival, and military pilots whoregularly fly at altitudes well above those of passenger air trafficwear their oxygen masks ready for use during the entire flight. There isno such necessity for civil aviation, because the cruising altitude ofairliners is lower, namely between FL 250 and FL 450, which correspondsto a flying altitude of about 25,000 ft to 40,000 ft.

For commercial reasons, flight routes should ideally be optimized to theshortest route between two airports. Nowadays, the ranges of modernairliners allow direct (inter)continental flights to destinations whichuntil a few years ago could only be reached with stopovers. However, forsafety reasons not all direct routes are open to air traffic. Flyingover high mountain ranges such as the Himalaya between India and Tibet,the Central Asian Hindu Kush and the South American Andes is onlypossible with restrictions and under certain conditions. The decisiverequirements are dictated by two distinct emergency situations: enginefailure on the one hand and loss of cabin pressure on the other. In thefirst case, the danger resides in the loss of thrust due to the failureof one or more engines, forcing it to reduce altitude, since an aircraftwith reduced thrust cannot maintain its cruising altitude. In suchcases, escape routes enable the aircraft to execute a drift-down to theclosest possible runway. The ICAO (International Civil AviationOrganization), the EASA (European Aviation Safety Agency), the JAA(Joint Aviation Authorities) and as well the FAA (U.S. Federal AviationAdministration) all prescribe that such emergency escape routes mustsatisfy a standard according to which a vertical clearance of at least2′000 ft from the ground must be assured during the engine-outdrift-down manoeuvre to the OEI (one-engine inoperable) service ceiling.and when level flight is re-established a vertical clearance of 1′000feet above the ground and 2′000 feet above mountains within a specificlateral distance relative to the flight path. The provisions of theregulatory authorities differ with regard to the specific lateral trackwidth for said obstacle clearance.

If emergency landings were caused by engine failure only, thesophisticated system of escape routes would allow for almost any directflight routes, notwithstanding some minor deviations. But the secondcase of a possible emergency situation, that is to say loss of cabinpressure, represents a substantially more restrictive set of problems.Besides the requirements mentioned above, the time factor is far moresignificant than in the case of engine failure. The number ofpotentially possible escape route is reduced considerably, because onmany such routes the required difference in altitude cannot be attainedwithin a sufficiently short time. The standard procedure in the event ofloss of cabin pressure is regulated by the ICAO. It provides that—aftersafely breathing through their own oxygen masks—pilots initiate adescent as quickly as possible to bring the aircraft to a saferaltitude, i.e. to a level at which humans can breathe without additionaloxygen supply. This must be accomplished within the bridging timepredefined by the oxygen supply. Due to limited space and weightcapacity of an aircraft, the oxygen supply can be increased only at thecost of cargo or the maximum number of passengers. An improvedventilation of the passengers in case of cabin depressurization mightthus also result in less oxygen having to be carried aboard theairplane, thus reducing the overall weight of the aircraft.

If the problem of artificial respiration did not exist, in many casesairliners would be able to fly to their destinations by a more directroute. If engine failure were the only limiting factor, multi-engineaircraft with passengers aboard would be able to overfly any region,because the escape route system allows for a drift-down to theone-engine inoperable service ceiling on any route section. In practicehowever, in order to qualify as emergency escape route, any suchoff-track route must meet the more restrictive set of requirements ofthe two above-mentioned emergency situations—i.e. that of spontaneouscabin depressurization. An improved ventilation system for passengers inthe event of cabin pressure loss which would require less oxygen couldextend the time interval imposed by a conventional oxygen supply.Consequently, it may not be necessary to implement safety precautionsexceeding those prescribed for the case of engine failure. Without theseadditional limitations, itineraries could be flown over high-altitudeterrain without detours and consequently thousands of tons of kerosenecould be saved. The saving in fuel weight would enable either theloading capacity to be increased or the overall fuel consumptionreduced, because the aircraft's weight is reduced by the deadweight ofthe reduced fuel requirement. This would also contribute toenvironmental conservation. Finally, flight times could be reducedconsiderably, offering not inconsiderable operational advantages as wellas direct flights to more distant destinations.

The flight routes that are actively flown over extensive mountainousareas today involve substantial planning effort, besides actuallycarrying them out. In the event of major technical problems andmalfunctions, it is primarily the pilots' responsibility to handle theemergency situation effectively, that is to take immediate decisions andimplement the necessary steps. Since decisions in such situations aremost often irreversible, the outcome of events is largely determined byhumans who must react in these stress situations. Thus, a significantrisk of errors with possibly fatal consequences is given a priori. Aless urgent time factor or a longer decision period in emergencysituations would increase the quality of decisions significantly and indoing so, contribute substantially to safety.

Therefore, it is the object of the present invention to provide aproduct and generally a method as well as a use of said product for thepurpose of ventilation at high density altitudes or in case ofhyperventilation, to ensure gas exchange with the human body and toimprove body function and performance by providing more efficientartificial respiration in situations where such becomes necessary. Theobject of the present invention is further to provide a product, amethod and use of said product which prolongs the period remainingbetween the instant when the oxygen is made accessible to the airpassengers and the instant when the aircraft has descended to a safeflight altitude corresponding to a high-density altitude in which humanscan survive, to such a degree that in case of cabin pressure loss, moretime is available to the pilots to take measures and decisionsrespectively for minimizing danger and increasing general flight safety.It is a further object of the invention to provide a product, a methodand the use of said product, so that a flight route need not necessarilycomply with more restrictive itinerary-related requirements than in caseof an engine failure scenario, such that an aircraft may descend moreslowly in the event of a depressurization occurring. This isadvantageous for itinerary-related reasons, as this allows more directflight routes over high-altitude terrain. It is still another object ofthe invention to provide a product, a method and the use of saidproduct, so that in the event of loss of cabin pressure the bodyfunctionality of the air passengers and crew is ensured using lessoxygen than previous ventilation systems, thereby reducing or preventingharmful risks associated with the supply of pure oxygen. In particular,the invention is intended to help effectively avoid the occurrence ofotherwise threatening symptoms for people in a situation such asprevails in an aircraft, i.e. in which physical movement is limited andsusceptibility to complications from artificial respiration with pureoxygen is thus increased.

Regarding the first aspect, the problem is solved with a gas mixture forensuring good ventilation of air passengers and crew in emergencysituations, or generally in cases of hyperventilation, which ischaracterized in that depending on density altitude it comprises 7±5%CO₂ at 15,000 ft flying altitude, increasing to 17±5% CO₂ at 30,000 ftflying altitude, to act as bioenhancer and thus to improve thebioavailability of oxygen in the body, by additive dosage of the carbondioxide to either pure O₂ or to a gas mixture comprising a fraction ofN₂ and a fraction of O₂ for ventilation.

The object of the invention is also solved with a method for ensuringventilation of people with limited mobility in emergency situations, orgenerally in cases of hyperventilation, in which the method ischaracterized in that artificial respiration masks are made availablefor placing over the nose and mouth, through which the gas mixtureaccording to one of the claim 1 or 2 is continuously supplied uponfitting the mask to nose and mouth of the respective person.

Thirdly, the object of the invention is solved by the use of a gasmixture of one according to one of the claim 1 or 2 for ensuring goodventilation of people with limited mobility if needed, or generally incase of hyperventilation.

The product, the method and the use of said product are disclosed on thebasis of the following explanations. The efficiency of the method hasbeen demonstrated in many trials and measurements. The content thereofwill be discussed in the following.

In the drawing

FIG. 1 : shows a flight route modified due to the highly elevatedterrain of the Bolivian-Argentinian Andean mountains;

FIG. 2 : shows the associated altitude profile of the flight route withthe required minimum ground clearance indicated by way of superpositionon the altitude profile;

FIG. 3 : shows a typical emergency descent profile of an aircraft to areduced flight altitude;

FIG. 4 : shows a flight route between La Paz and San Salvador de Jujuywith an exemplary system of escape routes;

FIG. 5 : shows a procedure with options A, B, C for descent in an enginefailure scenario;

FIG. 6 : shows a 737-700 CFM56-7 emergency descent profile in the eventof loss of cabin pressure for a standard descent in 15 minutes oroptional descent in 22 minutes, respectively, provided by Boeing inaccordance with the regulations set by ICAO;

FIG. 7 : shows descent options according to a 12-minute descent profilefor an aircraft in the high mountainous area around La Paz;

FIG. 8 : shows descent options according to a 22-minute descent profilefor an aircraft in the high mountainous area around La Paz.

FIG. 1 shows the implications of the constraints imposed on routeplanning due to the problem of ensuring adequate oxygen supply. This isillustrated by means of a flight route between Panama City and BuenosAires. In the case of direct route, after about halfway the flight pathpasses over the Bolivian and subsequently the Argentinian Andes.Accordingly, this extensive and extremely high mountainous areaconstitutes a restriction on emergency descent options, because in theevent of cabin depressurization over the extensive high plateau, theaircraft is unable to descend to a safe altitude where air passengerscan breathe autonomously within the required time limit. For thisreason, a substantially longer detour route 1 via Santa Cruz needs to betaken mostly over lowland, avoiding the critical Andes mountains. Theeconomical and efficient variant of a direct route 2 therefore must notbe used. The altitude profile associated with a direct route 2 betweenPanama City and Buenos Aires is represented in FIG. 2 . The diagramshows two curves 3 and 4, wherein the lower curve 4 indicates thealtitude profile corresponding to the geography of the terrain.Superimposed on this is the minimum vertical clearance 3 which anaircraft must observe at every point of the flight route. The prescribeddescent profiles must therefore be higher than the mandatory groundclearance.

FIG. 3 illustrates a typical emergency descent profile of an aircraftfollowing an engine failure. The actual trajectory 6 (net flight path)as well as the idealized trajectory 7 (gross flight path) as developedin flight route planning are indicated. In the event of engine failureduring flight, a positive climb gradient must be attained afterdrift-down 5, when the airplane is at least 1,500 ft above the(emergency) landing site. The positive climb gradient is indicated inFIG. 3 at the lowest point 8 of the flight path (at least 2000 ft ofvertical clearance above the ground or 1000 ft of vertical clearance inthe level-off segment of lower flight routes). If a fully loadedaircraft cannot fulfil the required standards at every stage of itsflight route, it is not permitted to fly that route.

An emergency escape route system complying with the regulatory safetyprovisions has been developed for all flight routes over high altitudemountains. An example of such a system is shown in FIG. 4 , whichrepresents the flight route over the Andes mountains between La Paz andSan Salvador de Jujuy. The filled area 15 marked with a dotted zigzagline indicates very high terrain, and the area 16 to the right of thesolid line indicates very low terrain, while the remaining white areasindicate moderate height. The territorial strip 15 along the flightroute depicts the particularly high elevation of the Andes massifbetween Bolivia and Argentina. Route planning is determined primarily bythe limited amount of oxygen aboard the airplane for supply to crew andpassengers mentioned in the introduction. A route along this massif,which would thus constantly be over the high altitude terrain wouldtherefore not be permitted. This is shown in FIG. 4 by the bold dottedline 9. The dashed, direct route from La Paz to San Salvador de Jujuyfor example can be flown instead. In practice, local andaviation-related factors such as wind, temperature, local pressure,weight etc. can also lead to minor deviations from a potential route andtherefore need to be determined specifically. It should be noted herethat aircraft which fly such a specific route as indicated by thisdashed straight are equipped with corresponding high-capacity gaseousemergency oxygen systems, as is explained in the following section. Onthe direct route between La Paz and San Salvador de Jujuy two ideal turnpoints 11 and 13 are shown, leading straight to the airports of Sucre orTarija. The possible emergency escape routes are either perpendicular tothe direct route (shortest emergency escape routes) or they representtwo sides of an equilateral triangle (longest emergency escape routes).This creates a triangular escape area useful for orientation, enablingidentification of the points marking the quickest possible descent(ideal turn points). The critical points 10, 12 and 14 that allow one orthe other landing option to be considered are positioned exactly halfwaybetween the “ideal turn points”. The decision regarding the routeeffectively selected for an emergency descent is made by the flightcaptain.

Some airliners and business jets are equipped with high-capacity gaseousemergency oxygen systems, also known as burning systems (due to the heatgeneration resulting from the chemical reaction, by means of whichoxygen can be produced aboard). Accordingly, a small number of airlinersthat cover long distances over high mountain regions are equipped withsuch high-capacity oxygen devices. However, the oxygen tanks and theessential equipment involved result in additional weight, which isdetrimental to flight performance especially in the event of an enginefailure, when only a reduced number of engines provide thrust. In such ascenario, the effective resulting OEI (one-engine inoperable) serviceceiling depends on a series of factors, including the number of enginesremaining operative, and particularly on the weight of the aircraft. Theflight team needs to strictly adapt the procedure of an emergencydrift-down to these conditions. FIG. 5 indicates the possible options.In each case, upon occurrence of an engine failure at point 18, as thefirst 1st measure the maximum continuous thrust is set immediately, asthe 2nd measure airspeed is reduced, and as a 3rd measure drift-down isinitiated at a defined drift-down speed. Depending on the situation, asa 4th measure a decision is then made regarding whether to adopt one ofthree options A, B or C, According to option A, airspeed is maintainedafter the drift-down and the aircraft climbs constantly to a higherflight altitude with continued fuel consumption (A). According to optionB, the flight altitude is maintained for the remainder of the flight andairspeed is gradually increased to engine out long-range cruise speed,or according to option C altitude is reduced airspeed is increasedimmediately to engine-out long-range cruise speed. If after adrift-down, the required height of the flight path, i.e. at least thealtitude required by option C, cannot be attained further along theairway due to high-altitude terrain, the aircraft's payload must bereduced, for example, by partially emptying or burn off of fuel, whichallows a higher flight altitude (option A). However, this requires thatan emergency landing site is available within a foreseeable distance.The aircraft weight is always a negative factor in an engine failurescenario. There is an inherent conflict between solving artificialrespiration problems in the event of cabin depressurization on the onehand, and the required drift-down profiles in case of engine failure onthe other. If the need for an efficient oxygen supply of air forpassengers and crew takes precedence due to the altitude profile of theterrain covered by a certain flight route, the weight disadvantage of amore efficient oxygen supply system is considered. The additional timeper meter of altitude thus gained in the event of cabin pressure lossallows longer escape routes, by which multiple off-track escape areasmay be accessed. On the other hand, if the weight disadvantage cannot becompensated any longer, the route must not be flown. This is usually thecase with routes covering extended high mountain areas.

In an event of cabin depressurization the specific provisions of ICAOapply. The specific descent profiles prescribed by different aircraftmanufacturers or airlines, respectively are derived therefrom. Ingeneral, the ICAO prescribes two different drift-down profiles, astandard descent profile and an optional descent profile for exceptionalroutes. Two such profiles with specific values determined by Boeing areshown in FIG. 6 , namely a 12-minute standard profile 20 and an optional22-minute profile 19. The magnitudes of these values are the same forall aircraft manufacturers and airlines. Following cabindepressurization, an aircraft of the manufacturer Boeing (The BoeingCompany) must have descended to 14,000 ft in 12 minutes or 22 minutesrespectively depending on its certification. The intermediate altitudesand intermediate times specified in the respective profiles 19, 20 alsohave to be observed. A flight route must be selected such that anaircraft can adhere to these altitudes and times throughout the flight.Thus, when flying over extensive high mountain ranges, detours arenecessary to be able to adopt with the prescribed descent profile andthus descend to a lower altitude quickly enough at all times if a lossof cabin pressure occurs. More direct flight routes over extensive highmountain areas can almost only be flown by cargo aircraft, because thesecarry a greater oxygen supply for the crew than airliners. As describedpreviously, the planned flight path must also allow for the regulatedprocedures in case of an engine failure, as previously described. Inthis case, the descent profile 7 of FIG. 3 is decisive. Modern airlinersare able to fly at considerably higher altitudes after an engine failurethan those specified by the profiles in case of cabin depressurization.As a consequence, the limitations imposed on potential flight routes areprimarily determined by a potential cabin pressure loss, specified bythe emergency descent profiles 19, 20 according to FIG. 6 . In general,all emergency escape routes satisfying the requirements in case of cabinpressure loss are equally suitable in the event of an engine failure,but conversely a flight route that is suitable for an engine failurescenario must meet the required time conditions determined by theemergency oxygen system aboard in order to qualify. This reduces thenumber of potential emergency escape routes considerably. As aconsequence, high-altitude areas like the Central Asian mountain regionsor the Andes are only open to limited passenger air traffic. FIG. 7shows how an aircraft appropriately certified with respect to its oxygensupply system for a descent within 12 minutes can descend over theBolivian Andes in an emergency situation. The requirements according tothe 12-minute profile make it impossible for the aircraft to land in LaPaz, as it cannot maintain the intermediate altitudes and intermediatetimes according to this profile. This is illustrated by the dashed line23 representing the descent profile passing east of the city of La Paz,showing that the line 23 passes below the minimum required groundclearance altitude 21, and even below the geographical profile 22 of theterrain. Thus, a descent is possible only in the westward direction,provided the aircraft adopts a position as indicated on thecorresponding descent line, or further to the west. For this reason, itis not possible for aircraft with certification for an emergency descentin 12 minutes to fly a route over La Paz, as shown in FIG. 7 by theaircraft positioned to the west. By contrast, in order to be able tooverfly this region, an aircraft must be equipped with a high-capacitygaseous emergency oxygen system, so it can descend in 22 minutes in caseof a cabin pressure loss, in accordance with its certification. Eventhen, the intermediate altitudes and intermediate times must beobserved. In FIG. 8 a critical point 24 is marked, in which therequirements for an emergency landing in La Paz can no longer be met. Ifan aircraft with certification according to a 22-minute profile is at analtitude of or even below the critical point 24, it can only follow anemergency escape route leading westward, since otherwise the route isblocked by mountain massif east of La Paz and the descent profile 23runs beneath the minimum required ground clearance and even below thegeographical altitude profile 22 for the terrain. In summary, it may beobserved that many limitations are imposed on air traffic due to theproblem of cabin depressurization.

In fact, the conventional cabin pressure loss scenario assumes fullyfunctioning engines, which in principle allows airspeeds higher than inthe event of engine failure. One might therefore expect that obstaclessuch as high-altitude terrain could more likely be overflown within theprescribed time interval, thus imposing less stringent requirements onroute planning. In reality, however, it must be assumed that cabindepressurization is caused initially by a structural failure, so thatthe airspeed must be adjusted immediately, that is to say reduced.Therefore, it is not possible to overfly the large expanses ofhigh-altitude terrain without restrictions and still comply with descentprofiles described above.

Oxygen undersupply to body tissue in healthy people is usuallyattributable to an O₂-poor environment. Probably, the greatest risk ofacute oxygen deficiency for an average healthy human is cabindepressurization in an aircraft. If cabin pressure drops unexpectedly athigh flight altitudes, the low partial pressure of oxygen leads to anundersupply of oxygen to body tissue (hypoxia). Hypoxia can result insevere organ damage, possibly even leading to death. One insidiouscharacteristic of hypoxia is that it is not always detected or isdetected too late by the person concerned, so he/she is already limitedin his/her ability to take corrective action. Symptoms of hypoxiainclude the spectrum from wrong self-assessment, euphoria, fatigue,disorientation, to unconsciousness. In aviation, hypoxia is consideredan extremely serious physical condition, which can have fatalconsequences, especially for the crew of an aircraft.

While supplying pure oxygen to humans, the partial pressure of oxygen isincreased five-fold. According to Henry's Law, the partial pressure of agas over a liquid is proportional to the concentration of the gas(physically) dissolved in this liquid. Thus, when supplying the bodywith pure oxygen, the proportion of the dissolved oxygen in the bloodincreases five-fold. On the other hand, the gas law does not apply tothe oxygen which is chemically bonded to the haemoglobin of red bloodcells. Under normal breathing conditions, the oxygen saturation of bloodalready amounts to 95-100%. Thus, during ventilation mainly theproportion of the physically dissolved oxygen is enriched. The latter isthen pressure-dependent. If a human inhales pure oxygen at anatmospheric pressure of 2.5 bar, 20 times the amount of oxygen isdissolved in blood compared to standard conditions. This “systematichyperbaric oxygenation therapy” is used when low blood oxygen in thebody tissue prevents the healing process of patients, or when oxygenmust be supplied as a life-saving measure in emergency situations.However, hyperbaric oxygenation has so far not found wide clinicalapplication, mainly because of the side effects of high oxygen contentand excess pressure. The oxygen therapy in intensive-care medicine isone of the main causes of oxygen toxicity damage.

The pressure of breathing air is a highly regulating parameter. It canalso exert a moderating influence when enriching blood with oxygen. Inthe case of continuous, controlled ventilation, as in space travel forexample, ventilation using pure oxygen must be operated at lowpressures, i.e. ambient pressure is not allowed to exceed 0.3 bar. Thus,the barometric pressure and consequently the oxygen partial pressure ofthe supplied respiratory air is reduced (cf. oxygen partial pressure of0.21 bar under normal pressure). With continuous ventilation with pureoxygen, the risk of oxygen intoxication is present as early as pressuresabove said value of 0.3 bar.

Ventilation with pure oxygen can be made possible for various scenariosif the ambient pressure is varied accordingly, but its conflictingcharacteristics mean that the oxygen cannot be supplied alone in thedoses required to counteract oxygen insufficiency without side effects.Surprisingly, it was discovered in experiments and test runs thatinhalation of a gas mixture comprising 7±5% CO₂ at 15′000 ft altitude,and increasing to 17±5% CO₂ at 30′000 ft altitude, vastly improves boththe physical and mental functionality of the body in a condition ofacute oxygen undersupply, compared to that resulting from pure oxygensupply. The improvement is unprecedented.

If an undersupply of oxygen occurs in the body, the body reacts byaccelerating the breathing rate. The increase in the breathing ratecauses more oxygen to be inhaled per unit time, but at the same timemore carbon dioxide is exhaled. In the body, carbon dioxide ischemically bound as carbonic acid (H₂CO₃). From the formula belowCO₂+2H₂O

HCO₃ ⁻+H₃O⁺

H₂CO₃+H₂Othe chemical balance indicates that reducing CO₂ in the body results inthe number of H₃O⁺-ions in the blood to be reduced equally. This causesa shift in the acid-base equilibrium, because the blood becomesincreasingly alkaline. In extreme cases, this results in a respiratoryalkalosis with symptoms of muscle cramps, impairment of consciousness,even loss of consciousness. Moreover, the increase in the pH-value ofblood effects a decrease in the concentration of freely dissolvedionized calcium (hypocalcaemia), leading to hyperexcitability of themusculature and nervous system, exhibiting spasmodic symptoms.Conversely, an increased concentration of carbon dioxide in the bloodshifts the pH-value of the blood into the acidic range. Carbondioxide-sensitive receptors are located on the vessels of many organs.Depending on the specific organ, the blood vessels either contract orexpand under the influence of carbon dioxide. The vessels of the brainexpand upon an increase of carbon dioxide concentration. The blood-flowrate increases and with it, the oxygen amount reaching the cells perunit time. In this way, the body attempts to compensate for the oxygenundersupply, and in particular, to supply the brain with sufficientoxygen for as long as possible. The opposite effect is observed, if thebody is supplied with high oxygen dosage, while reducing the carbondioxide supply. A hypocapnia, i.e. a low carbon dioxide partial pressurein the arterial blood, leads to contraction of blood vessels in thebrain and consequently reduces of the blood and oxygen supply. Whencabin pressure is lost in an aircraft, an undersupply of oxygen to thebody occurs. The body begins to hyperventilate. Even if a passengerreaches quickly for the artificial respiration mask, the tendency tohyperventilate is further increased by the stress-induced circumstances.Hyperventilation accelerates the rate at which carbon dioxide isexhaled. This reduces the level of carbon dioxide in the body. Since theventilated air passenger's mobility is limited due to the circumstances,less carbon dioxide is produced by the muscular cells and the effect ofthe carbon dioxide deficiency is accelerated accordingly. For airpassengers, who are restricted to their seats for most of the time, andmost particularly in emergency situations, this limitation of mobilitymay have severe consequences, because the body then produces less carbondioxide. Not least because of this fact, a rapid descent to a safeflight altitude is essential for survival.

If a dosed, pressure-dependent amount of carbon dioxide is added to thebreathing gas, the effects as mentioned in the section above can bediminished. As the active supply of carbon dioxide to the body relaxesblood vessels in the brain, the oxygen supply to the body tissue takesplace in a more efficient manner, while at the same time the amount ofoxygen is reduced. Oxygen is then reabsorbed more quickly and to agreater extent, and so provided to the tissue and cells, respectively.The gas mixture according to the invention ensures respiration inemergency situations and in doing so, increases the bioavailability ofoxygen, in particular oral bioavailability, because carbon dioxide inprecisely measured doses acts as a bioenhancer. Finally, because of thegas mixture according to the invention the body is kept at aphysiological level of carbon dioxide with just a partial dose of oxygenand over a substantially longer period. This provides significantadvantages, particularly in cases of depressurization of aircraftcabins.

Surprisingly, aeromedical experiments have demonstrated that byinhalation of air enriched with carbon dioxide aviation standard valuescan be attained: for a bridging time of maximally one minute, 84% oxygensaturation of the blood is prescribed, and for bridging time lastingmore than one minute, 90% oxygen saturation of the blood is required.For the experiments, test persons were administered the amount of carbondioxide required to maintain the carbon dioxide level in the blood at apartial pressure of 40 mmHg at different density altitudes. Therespiratory air was prepared such that it comprised the followingamounts of carbon dioxide at distinct density altitudes: 8% CO₂ at15,000 ft, 11% CO₂ at 20,000 ft, and 16.5% CO₂ at 30,000 ft. Theaddition of carbon dioxide to the gas mixture was at the expense ofnitrogen. Consequently, the gas mixture for ventilation was composed asfollows:

At 15,000 ft density altitude: 21% O₂, 8% CO₂, 71% N₂

At 30,000 ft density altitude: 21% O₂, 16.5% CO₂, 62.5% N₂

Each test person had to undergo two simulated emergency descent profilesfrom 37,000 ft to 10,000 ft altitude, the descent corresponding to theprofiles specified by the ICAO. During the one descent, the test personsinhaled pure oxygen as is usually the case in a cabin pressure lossscenario, and during the other descent, the same test persons inhaled agas mixture with carbon dioxide added as described above. The experimentwas structured in a randomized, double-blind protocol. Neither observersnor test persons knew which gas mixture would be supplied in whichdescent. The results indicate the following key benefits:

-   -   1. The amount of cabin oxygen aboard an aircraft can be reduced.    -   2. Based on the adapted drift-down procedures, more direct        flight routes can be flown, thus saving essential costs and        time.    -   3. Since—as a consequence of the above—less onboard fuel is        needed, the cargo capacity of the aircraft increases.    -   4. Owing to the reduced fuel consumption the environment is        protected.

A particularly elegant aspect of the overall approach regarding themethod of additive dosing of carbon dioxide according to the inventionis that a passenger himself produces at least part of the requiredcarbon dioxide and oxygen for the purpose of ventilation, respectively.Breathing air under normal ambient pressure consists of approx. 78%nitrogen (N₂), 21% oxygen (O₂) and approx. 1% residual gases. Bycontrast, exhaled air consists of approx. 78% nitrogen (N₂), 16% oxygen(O₂), 4% carbon dioxide (CO₂) and approx. 2% residual gases. The exhaledamount of carbon dioxide and oxygen can be recovered. Dosing of the gasmixture according to the invention can thus be accomplished using carbondioxide and oxygen supplied by the ventilated person him/herself, whichis then eventually inhaled again by him/her, while the remaining gasfraction is added synthetically. The higher the density altitude, themore carbon dioxide needs to be dosed additively at the expense ofnitrogen.

LIST OF REFERENCE NUMERALS

-   1 Actual flight route-   2 Direct route which may not be flown-   3 Minimum required ground clearance-   4 Terrain elevation profile-   5 Drift-down-   6 Gross flight path-   7 Net flight path-   8 Positive climb gradient begins-   9 Route which may not be flown-   10 Critical point 1-   11 Ideal turn point 1-   12 Critical point 2-   13 Ideal turn point 2-   14 Critical point 3-   15 High mountain region-   16 Lowland-   17 South American west coast-   18 Engine failure-   19 Optional 22-Minute System-   20 Standard 12-Minute System-   21 Minimum required vertical clearance-   22 Terrain elevation profile-   23 Course of the descent profile curve below the terrain elevation    profile 22-   24 Critical point    Checklist Points (FIG. 5 )    -   1. Adjust maximum continuous thrust (MCT)    -   2. Maintain altitude, decelerate to drift-down speed    -   3. Maintain drift-down speed    -   4. Select one of the three drift-down options:        -   A: Maintain airspeed and climb until fuel burns off        -   B: Maintain level flight and accelerate to EOLRC speed            gradually (EOLRC=engine-out long-range cruise speed)        -   C: Descend and accelerate to EOLRC speed immediately            (EOLRC=engine-out long-range cruise speed)

The invention claimed is:
 1. A method of ventilating an air passenger inan aircraft, comprising providing a flow of a gas mixture through aventilation mask, wherein the gas mixture is enriched with CO₂ relativeto air at sea level, and wherein the amount of enriched CO₂ varies from7±5% CO2 at 15,000 ft flight altitude to 17±5% CO2 at 30,000 ft flightaltitude.
 2. The method according to claim 1, wherein the enriched gasmixture is obtained in part using carbon dioxide and oxygen supplied bythe air passenger.
 3. The method according to claim 1, wherein the gasmixture is also enriched with O₂ relative to air at sea level.
 4. Amethod of ventilating an air passenger in an aircraft, comprisingproviding a flow of a gas mixture to act as a bioenhancer and improvethe bioavailability of oxygen in the body, wherein the gas mixture isenriched with CO₂ relative to air at sea level, and wherein the amountof enriched CO₂ comprises 7±5% CO₂ at 15,000 ft flying altitude,increasing to 17±5% CO₂ at 30,000 ft flying altitude.
 5. The methodaccording to claim 4, wherein the gas mixture is delivered to the airpassenger through a respiration mask.
 6. The method according to claim4, comprising adding carbon dioxide to pure O₂ or to a mixturecomprising a fraction of N₂ and a fraction of O₂ for ventilation.